1. Field of the Invention
The present invention relates to a data communication system and, more particularly, to a data communication system having an error correcting function.
2. Description of the Related Art
Data communication systems such as facsimiles have been known in which received image data is recorded in real time and, upon safe receipt of one page of image data, a message acknowledgment signal indicative of the safe receipt of the data is issued.
Apparatus are also known in which the received data is first stored in a memory and then recorded and a message acknowledgment signal is produced when storage of one page of image data is finished. In this type of apparatus, the message acknowledgment signal is issued before the recording of the received image data.
The apparatus of the second-mentioned type is therefore disadvantageous in that the message acknowledgment signal is wrongly sent to the transmitting or sending side even when the recording is not actually conducted due to, for example, a power failure which has taken place after the receipt and storage of the image data. In consequence, the sending apparatus may wrongly understand that the recording at the receiving side has been safely completed. Thus, the image data may be lost if the operator of the receiving side is not aware of the recording failure.
Known facsimile apparatus also suffers from a problem in that, when a training receipt of a high-speed signal (image signal) fails during communication in G3 mode, the communication is undesirably suspended. If the training receipt of the high-speed (image) signal is completed successfully, the facsimile apparatus starts to receive the image data. If an RTC (Return to Control) signal could not be received during receiving of the data due to, for example, a disturbance of the data, the facsimile apparatus undesirably remains in the mode for receiving high-speed signal. In such a case, triple transmission of Q (MPS or EOM or EOP) is finished while a decoding circuit is seeking for the RTC, with the result that further transmission becomes impossible.
Known facsimile apparatus having an error correction function has encountered a problem in that an error frame often remains even when error frames have been sent PG,5 repeatedly after selection of the error correction mode at the receiving side. In such a case, an error is undesirably caused in the recorded image although the error correction mode has been selected. This is quite inconvenient considering that the operator at the receiving side, who has selected the error correction mode, is convinced of safe receipt of data without error.
In order to overcome this problem, the present applicant has proposed, in the U.S. patent application Ser. No. 267,541, a data communication system in which the recording of an error frame is neglected or replaced with a unique code, in the event that such an error frame remains. This apparatus, however, cannot enable an operator to check whether any error frame exists in the received data by an at-a-glance check.
When data on a long continuous sheet is sent while the facsimile apparatus of the receiving side is of the type which records received data on cut sheets, one physical page transmitted from the sending apparatus is recorded in a plurality of, e.g., three, physical pages at the receiving side. If an uncorrectable frame exists in the first physical page, the data of this frame is neglected or replaced with a unique code, with the result that the positions of recording on the second and third physical pages are undesirably shifted from the positions where the data is to be recorded when there is no error frame. In addition, when cut-sheet type apparatus is used at the receiving side, the operator at the receiving side is often confused because there is no means for determining whether the received data is a part of data on a continuous long material or whether one physical page at the receiving side corresponds to one physical page transmitted from the sending side. Thus, the operator may erroneously understand that the data recorded on the above-mentioned second and third pages have been received correctly, without being aware of the omission of the error frame.
Although data communication systems having an error correction mode have been proposed in U.S. patent application Ser. Nos. 110,542, 847,684 and 162,266, none of such applications proposes a measure for overcoming the above-described problems.